linux-kernel - Re: [PATCH 5/5] kvm: optimize GFN to memslot lookup with large slots amount

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Date:	Tue, 2 Dec 2014 18:33:51 +0100
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, pbonzini@...hat.com,
	kvm@...r.kernel.org
Subject: Re: [PATCH 5/5] kvm: optimize GFN to memslot lookup with large slots
 amount

2014-12-01 17:29+0000, Igor Mammedov:
> Current linear search doesn't scale well when
> large amount of memslots is used and looked up slot
> is not in the beginning memslots array.
> Taking in account that memslots don't overlap, it's
> possible to switch sorting order of memslots array from
> 'npages' to 'base_gfn' and use binary search for
> memslot lookup by GFN.
> 
> As result of switching to binary search lookup times
> are reduced with large amount of memslots.
> 
> Following is a table of search_memslot() cycles
> during WS2008R2 guest boot.
> 
>                          boot,          boot + ~10 min
>                          mostly same    of using it,
>                          slot lookup    randomized lookup
>                 max      average        average
>                 cycles   cycles         cycles
> 
> 13 slots      : 1450       28           30
> 
> 13 slots      : 1400       30           40
> binary search
> 
> 117 slots     : 13000      30           460
> 
> 117 slots     : 2000       35           180
> binary search
> 
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---

Fast ... it looks that we don't even want to transfort the list-in-array
into a tree-in-array to have multiplication instead of division.

Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>
(Actually, all patches.)

>  include/linux/kvm_host.h | 34 ++++++++++++++++++++++------------
>  virt/kvm/kvm_main.c      |  8 +++++++-
>  2 files changed, 29 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 1a37144..193bca6 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -354,6 +354,7 @@ struct kvm_memslots {
>  	/* The mapping table from slot id to the index in memslots[]. */
>  	short id_to_index[KVM_MEM_SLOTS_NUM];
>  	atomic_t lru_slot;
> +	int used_slots;
>  };
>  
>  struct kvm {
> @@ -791,19 +792,28 @@ static inline void kvm_guest_exit(void)
>  static inline struct kvm_memory_slot *
>  search_memslots(struct kvm_memslots *slots, gfn_t gfn)
>  {
> +	int start = 0, end = slots->used_slots;
>  	int slot = atomic_read(&slots->lru_slot);
> -	struct kvm_memory_slot *memslot = &slots->memslots[slot];
> -
> -	if (gfn >= memslot->base_gfn &&
> -	    gfn < memslot->base_gfn + memslot->npages)
> -		return memslot;
> -
> -	kvm_for_each_memslot(memslot, slots)
> -		if (gfn >= memslot->base_gfn &&
> -		      gfn < memslot->base_gfn + memslot->npages) {
> -			atomic_set(&slots->lru_slot, memslot - slots->memslots);
> -			return memslot;
> -		}
> +	struct kvm_memory_slot *memslots = slots->memslots;
> +
> +	if (gfn >= memslots[slot].base_gfn &&
> +	    gfn < memslots[slot].base_gfn + memslots[slot].npages)
> +		return &memslots[slot];
> +
> +	while (start < end) {
> +		slot = start + (end - start) / 2;
> +
> +		if (gfn >= memslots[slot].base_gfn)

(Even thought division is costly, I think that checking here if 'slot'
 is the one we want wouldn't help very much.)

> +			end = slot;
> +		else
> +			start = slot + 1;
> +	}
> +
> +	if (gfn >= memslots[start].base_gfn &&
> +	    gfn < memslots[start].base_gfn + memslots[start].npages) {
> +		atomic_set(&slots->lru_slot, start);
> +		return &memslots[start];
> +	}
>  
>  	return NULL;
>  }
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 162817f..759af659 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -679,8 +679,14 @@ static void update_memslots(struct kvm_memslots *slots,
>  	struct kvm_memory_slot *mslots = slots->memslots;
>  
>  	WARN_ON(mslots[i].id != id);
> -	if (!new->npages)
> +	if (!new->npages) {
>  		new->base_gfn = 0;
> +		if (mslots[i].npages)
> +			slots->used_slots--;
> +	} else {
> +		if (!mslots[i].npages)
> +			slots->used_slots++;
> +	}
>  
>  	while (i < KVM_MEM_SLOTS_NUM - 1 &&
>  	       new->base_gfn <= mslots[i + 1].base_gfn) {
> -- 
> 1.8.3.1
> 
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